1. Field of the Invention
The present invention relates to an automatic gain controller (AGC) module of a wireless receiver, for example an IEEE 802.11a based Orthogonal Frequency Division Multiplexing (OFDM) receiver.
2. Background Art
Local area networks historically have used a network cable or other media to link stations on a network. Newer wireless technologies are being developed to utilize OFDM modulation techniques for wireless local area networking applications, including wireless LANs (i.e., wireless infrastructures having fixed access points), mobile ad hoc networks, etc. In particular, the IEEE Standard 802.11a, entitled “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band”, specifies an OFDM PHY for a wireless LAN with data payload communication capabilities of up to 54 Mbps. The IEEE 802.11a Standard specifies a PHY system that uses fifty-two (52) subcarrier frequencies that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM), or 64-QAM.
Hence, the IEEE Standard 802.11a specifies an OFDM PHY that provides high speed wireless data transmission with multiple techniques for minimizing data errors.
A particular concern in implementing an IEEE 802.11 based OFDM PHY in hardware involves providing a cost-effective, compact device the can be implemented in smaller wireless devices. Hence, implementation concerns typically involve cost, device size, and device complexity.
In particular, Automatic Gain Control (AGC) algorithms are used to ensure that a received wireless signal is amplified to the match the dynamic range of the analog-to-digital (A/D) converter of the receiver. Communications systems typically use analog AGC modules to control the received wireless signal detected by the receiver antenna. In particular, the AGC would control an analog amplifier and determine the peak signal levels of the amplified signal relative to the input range of the A/D converter; if the if the peak of the amplified signal exceeds the input range of the A/D converter, the AGC module reduces the gain of the analog amplifier using a control feedback system. However, substantial delays are introduced by the time necessary to measure the signal, and adjust the gain relative to the feedback system. These problems are substantial in the case of a digital wireless signal transmitted according to IEEE 802.11a, since the received signal may have a range of about from −90 dBm to −30 dBm. Hence, difficulties arise in providing a stable, feedback control system for an analog AGC module.